Okay, This will be the easiest, but slightly more dangerouse way to change font, because it writes to flash0, but there is no complicated system.dreg crap.
First get this version of flashmod here, thanks to Ravine for this modified version.
Take that, put it in your GAME folder, now, in your root, creat a folder called "flashmod", without the quotes. Now, download a premade converted psp font, rename it to "ltn0" without the quotes again, add .pgf if you have view extensions on. (in capital letters thats LTN0.pgf)
Now, run flash mod, select XMB, and go all the way down to install new fonts, and press X, now, the bar at the bottom will flash once, and then say failed, DO NOT WORRY, it says failed because there arent the 20 other files, your psp is still OK. Now exit flashmod, and your font should already be there.
I have tested this out with my psp, and it worked, and i love my font
Just incase something did in the 1% possibility something bad happens, im not responsible for anything, HAPPY FONTS!
ALSO, if your not satisfied, then flashmods last option is restore default fonts, and if you do that, it will restore the default fonts.